cadastral applications, the distance to the remote receiver
should not be more than 15-20 km. The only potential error
source which has not been extensively studied are
differential rotations between the INS and the remote
sensing device. In other words, to achieve the high attitude
accuracy, flexing of the aircraft hull, as well as vibration
differences at the two locations have to be investigated. In
principle, this amounts to the question whether or not the
: : b : : t :
rotation matrix dR, can be considered as time invariant or
not. If the differential rotations are well below the noise
level of 15"-30", current high precision INS together with
DGPS can be used to meet the attitude requirements of even
the most demanding applications discussed in this paper.
The determination of the translation vector drP is not as
critical. Even if the length of the vector changes somewhat
in time, it will usually stay well within the accuracy level of
5-10 cm. If photogrammetric cameras are used for high
precision applications and a block adjustment is possible,
the georeferencing problem can be reduced to precise
positioning of the aircraft. This can be done with sufficient
accuracy by using differential GPS and carrier phase
techniques. In this case, the costs for the georeferencing
system can be considerably reduced.
If high accuracy applications are excluded, more economic
solutions to the georeferencing problem can be found. Such
systems would be suitable for the bulk of applications in the
resource sector. They will therefore most likely be the
standard georeferencing systems of the future.Two solutions
seem to be especially attractive. One is completely based on
GPS technology, the other is a GPS/INS integration with a
lower cost INS.
The GPS solution would combine two narrow-correlator C/A
code receivers for positioning with a GPS multi-antenna
system for attitude determination. The system would be low
cost and has the advantage that the same receivers can be
used for both tasks and sufficient redundancy can be built
into the system design. Drawbacks are system stability and
data rate. To get full attitude resolution, one or two of the
antennas have to be installed on the wings. The resulting
attitude is affected by wing flutter and most likely deviates
considerably from the attitude of the remote sensing system.
This problem is aggravated by the fact that a 50 Hz output
rate is needed. Current GPS output rates are at 2 Hz, thus
interpolation is needed. The compounded influence of the
two effects has to be investigated to confirm the suitability
of such a system for the stated accuracy requirements. The
system would not be suitable for SAR-type applications
because the short-term velocity accuracy is not sufficient.
The medium cost GPS/INS integration solves the above
problems and has sufficient short-term velocity resolution
to also be used in SAR applications. It covers therefore a
rather broad range of applications. Since the INS can be
mounted on the same frame or the same platform as the
imaging system, there is no problem with system stability,
as long as the rigidity of the frame under vibrations is
carefully checked. Output rate is not a problem because
inertial systems come with rates of 50-100 Hz anyway. A
further advantage is that an onboard INS can usually be
modified to form part of the integrated system. This will
reduce the costs considerably. It appears therefore that the
development of such a system offers the best balance
between economy and technological risk. This is one of the
major reasons why the prototype development at the U of C
moved in this direction.
7. TESTING OF THE U OF C PROTOTYPE
SYSTEM
The current prototype system at the U of C consists of a
strapdown INS of the medium accuracy class (Litton LTN
90/100) and two Ashtech P XII receivers. This system was
recently tested with the Compact Airborne Spectrographic
Imager (casi) developed by Itres Research Ltd for this
project (Babey & Anger, 1989). The casi is a pushbroom
sensor that acquires one scanline at a time as it travels along
the flight line. The resulting image possesses a different set
of position and orientation parameters for each scanline, and
it often contains large distortions induced by movements on
the aircraft. An example of such an image is shown in Figure
3:
The calibration parameters in Equation (5) are f, Xp: Jp Xy,
b
and dR, and drb. Al parameters, except drP were
determined from a calibration target array in the area flown,
using a self-calibrating bundle adjustment. The orientation
differences between INS and the imaging sensor, an? will
change each time the imager and INS is installed in the
b net
aircraft. In fact, dR, may change during flight if the camera
is not rigidly secured with respect to the INS. However,
b ; : :
dR, can be calibrated from the imagery if three or more
control points are present in the block. A block is defined
by a series of overlapping flightlines. For convenience four
points were chosen for calibration. A bundle adjustment is
used to solve for these parameters, Gibson & Buchheit
(1990).
To fully geocorrect the imagery, Cosandier et al, (1992) and
create an ortho-image, the ground height must be taken into
account via a digital elevation model (DEM). The
integration and extraction of a DEM from casi is currently
being developed.
Figure 3: Raw Pushbroom Image
198
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